Pulsed laser light is used in a number of applications, e.g., in integrated circuit photolithography to expose photoresist on wafers by passing the light through a mask. This pulsed laser light can be generated using a gas discharge medium in a chamber by providing gas discharge between a pair of electrodes at very high voltages in very short electrical discharges in the gas discharge medium.
If the gas discharge medium contains fluorine, e.g., in an ArF laser system, then a fluorine-containing plasma will be generated between the pair of electrodes during operation. Fluorine-containing plasmas are highly corrosive to metals. Consequently, the electrodes will corrode over time during operation of the chamber. If the corrosion product either vaporizes or flakes off of the electrodes, then the situation is tolerable because steps can be taken to deal with the problems caused by this type of corrosion of the electrodes. What sometimes happens, however, is that localized buildups of the metal fluoride corrosion product form at various spots over the surfaces of the electrodes, predominantly on the surface of the anode because fluorine tends to follow the flow of current from the cathode to the anode. This localized buildup of the metal fluoride corrosion product is sometimes referred to as the formation of a “reef layer” or “reefing” due to the similarity in appearance of the corrosion spots to coral reefs. The spots at which reefing occurs on an electrode stick up into the plasma more so than the rest of the surface of the electrode. As such, reefing can cause arcing to occur in the plasma.
Arcing in the plasma is undesirable because it robs the laser chamber of energy because energy goes into the arc discharge rather than into to the laser cavity. Thus, when a significant amount of arcing occurs in the plasma, the electrodes must be replaced to keep the laser chamber operating efficiently. As such, reefing shortens the lifetime during which electrodes can be used effectively in a laser chamber.
It is in this context that embodiments arise.